The invention relates to a display tube comprising, in an evacuated envelope, an electron gun system for generating and focusing on a display screen (by means of a focusing lens) at least two electron beams. The electron beams are deflected by deflection means and describe a frame on the display screen.
Such a display tube is known from U.S. Pat. No. 4,301,389 in which a matrix of individually controllable electron sources is used which generate a number of electron beams. Such a multi-beam display tube may be used as a projection television display tube because a larger beam current can be combined with a larger resolving power as compared with a monobeam display tube. It may also be used, however, as a D.G.D. tube (D.G.D=Data Graphic Display) or as a tube having a large display velocity for displaying computer data. Lens defects of the focusing lens, for example, spherical aberration, astigmatism, coma and field curvature enlarge the spot of an electron beam on the display screen of the tube. When using a number of electron sources in one row or in one plane it is very difficult to obtain a number of identical spots on the display screen, because the influence of the lens defects increases as the distance to the axis of the focusing lens increases.